1. Field of the Invention
This invention relates to an ignition timing control system for internal combustion engines, and more particularly to an ignition timing control system for internal combustion engines which is capable of conducting ignition timing retard control so as to enable securement of adequate ignition energy in all engine operation regions and specifically to enable securement of adequate ignition energy for eliminating any danger of misfirings during times of engine speed change such as during acceleration and during high speed engine operation.
2. Description of the Prior Art
Recent years have seen the proposal of a number of internal combustion engine ignition timing control systems which include ar ignition timing retard adjustment that is activated during acceleration or the like for retarding the ignition timing in crank angle degrees so as to reduce engine output and thus suppress vehicle vibration. The assignee, for example, applied for patent on such a system under Japanese Patent Application No. 1(1990)-45373 filed on Apr. 18, 1989.
The limit value of the ignition timing on the retarded side is determined by the relationship between the required voltage (the voltage necessary for breakdown of the insulation (air gap) between the electrodes of the spark plug) and the generated voltage (the voltage generated by the ignition coil). Specifically, as shown in FIG. 7, the limit .theta..sub.igLGG on the ignition timing retard side centers on a battery voltage of 14.3 V and is set to increase in proportion as the engine speed increases. Therefore, if the ignition timing should be retarded when the engine speed is high or when the battery voltage is low, it may be impossible to provide the required voltage, which may, in the worst case, result in misfirings.
More specifically, where the TDC (top dead center) interval is represented by T as indicated in FIG. 8 and the dwell angle of the ignition coil, that is the time during which the current control transistor of the primary coil is on, is represented by t, the ON duty ratio (hereinafter sometime called simply the "duty ratio") can be represented by t/T. So as to secure the energy required for ignition when the battery voltage V.sub.B decreases, the ordinary practice is to establish a compensation coefficient K as shown in FIG. 9 and to increase the conductive period of the transistor to that obtained by multiplying the duty ratio (t/T) by the coefficient. As shown in FIG. 10, when the decrease in battery voltage becomes large, the duty ratio also becomes large, with the result that the amount by which the ignition timing is retarded increases, increasing the likelihood of misfirings. More specifically, as will be understood from FIG. 10, the shortening of the ignition period (TDC interval) with increasing engine speed makes it difficult to secure the required conductive period, while, as can be seen in FIG. 11, the required voltage grows greater with respect to the generated voltage with increasing proximity of the crank angle to TDC. This is because it is a general rule that the insulation breakdown voltage increases with increasing compression ratio in the combustion chamber, and since retarding the ignition timing is tantamount to shifting the ignition timing toward the compression point, this means that the required voltage becomes higher. Therefore, if retard adjustment, which tends to increase the required voltage, should be carried out when the decrease in battery voltage becomes large or the engine speed becomes high, there may in some cases occur misfirings. Since at this time the aforesaid retard control is implemented in view of factors on the power transmission side, if conducted during high engine speed operation, as during acceleration or the like, it may make it impossible for the generated voltage to satisfy the required voltage and, as a result, may in some cases lead to misfirings.